It is desirable to be able to quantify parameters of a road surface. Users of roads generally desire that the road is “smooth” and that it offers sufficient friction for them to be able to accelerate and brake safely. Furthermore the road surface has to offer sufficient friction to a vehicle tire when the road is wet.
In order to measure “smoothness” it is known to use vehicles travelling at relatively high speed such that the vehicle body and its suspension carry a form of inertial platform. The vehicle body directs a pulse of light from a laser towards the road surface and views the diffuse scatter from the surface with an optical detector which has a direction of view inclined with respect to the direction of the beam of laser light. This allows a direct measurement of the distance from the light source to the road surface to be measured. The vehicle body also carries an accelerometer aligned with direction of the projection of the laser and travels sufficiently fast such that vertical movements of the vehicle body can also be measured. When the displacement derived from the accelerometer is subtracted from the laser displacement, the displacement created by the road smoothness remains. This technique can only be successfully employed when the survey vehicle can travel sufficiently fast to allow the accelerometer to accurately measure vehicle body height variations. This technique cannot be used over short distances such as transverse or oblique to the direction of a carriageway.
Roads also need a rough or “textured” surface to allow water to be dispersed away from the contact area between the road and a tire. Failure to provide a textured surface prevents water from being dispersed and can lead to “aquaplaning” where the tire rides on a film of water above the road surface and loses friction contact with the road.
The surface texture can also be measured by directing a pulse of light onto the road surface and viewing diffuse scatter from the road surface at an oblique angle such that the distance between the road surface and the light source is translated directly into position on a photo-detector array. The difference between smoothness and texture is purely one of wavelength. Thus the “inertial platform” described hereinabove can also capture surface texture.